The heat treatment of semi-finished steel products is undertaken in particular for austenitizing purposes for press-hardening preparation. By means of austenitizing with subsequent press hardening, i.e. the deformation and quenching of the semi-finished steel product heated to approximately 800-1000° C., the formed semi-finished steel product has a martensitic structure, as a result of which the strength of the formed semi-finished steel product significantly increases.
In order to protect the semi-finished steel products from corrosion and oxidation during transport and during the heat treatment, it is known to provide said semi-finished steel products with an AlSi coating. For example, it is furthermore known from DE 10 2011 051 270 A1 that said coating first of all melts from the solid state during austenitizing and subsequently alloys into the basic steel material of the semi-finished steel products. It is furthermore known from said prior art that the coating material is deposited on the rollers used as the conveyer elements.
It is furthermore known from the described prior art to use mullite as the ceramic material for the rollers. If the Al—Si coating melts during the heat treatment, a liquid to viscous Al—Si layer forms on the mullite rollers in a region dependent on the heating profile. Since, during the heat treatment, the Al—Si alloys up with the iron of the semi-finished steel product, said AlSi layer arises on the ceramic rollers within a characteristic temperature and time range. Which conveyer elements are affected by the described deposits depends, firstly, as already described, on the temperature and time profile and, secondly, on the conveyer element surface material which comes into contact with the surface of the semi-finished steel products.
As an alternative to the coating of the semi-finished steel products with AlSi, zinc alloy platings, for example platings with a zinc and nickel coating, are furthermore known. Such semi-finished steel products with a zinc-based coating also have to be austenitized for press-hardening preparation.
Both the AlSi-coated semi-finished steel products and the semi-finished steel products with a zinc-based coating are suitable for the direct press hardening. In the direct press hardening, a blank is punched out of a coil and supplied for heat treatment without prior deformation. By contrast, in the “indirect method”, a blank is punched out of a coil, and subsequently cold-worked, and the preformed component is then supplied for heat treatment likewise for austenitizing purposes. In the direct method, which is preferred because of the one-step nature thereof, intensive contact between the surface of the blank and the conveyer elements of the heat treatment apparatus occurs during the transport of the flat blank. By contrast, in the indirect method, the preformed components are generally transported on work piece carriers during the heat treatment for austenitizing purposes. In this case, there is therefore generally no contact between the coated semi-finished steel product and the conveyer elements.
If then, during the direct method, alternating campaigns with AlSi-coated semi-finished steel products and semi-finished steel products with a zinc-based coating are conducted, interactions of the AlSi deposits on the conveyer elements with the zinc-based coating of the correspondingly coated semi-finished steel products occur. There are formed here, inter alia, low-melting phases, for example zinc and aluminum phases, which may lead to cracking on the component. During such alternating campaigns, it is also possible, on the surface of the austenitized semi-finished steel products, to determine Zn—Al particles, for example, which may both result in a higher degree of wear of the pressing tool and, furthermore, may influence the surface quality and varnish adhesion of the component.